Add logging relation

[rgildein]

Added logging relation and cos integration tests from chisme.

fixes: #194

This was also manually tests with pre-deployed cos bundle.

juju consume u1-k8s:cos.loki-logging 
juju relate loki-logging grafana-agent-k8s

[rgildein]

Left a comment, this looks good. Could you update also the integration tests that fail?

I forgot to push changes waiting only for related apps in test_relations tests, since grafana-agent-k8s is in block state.